This invention relates generally to ultrasound transducers, and more particularly, to acoustical stacks that are within the ultrasound transducers.
Ultrasound transducers (also commonly referred to as probes) typically have many acoustical stacks arranged in one dimension or in two-dimensional (2D) arrays. Each acoustical stack corresponds to an element within the transducer, and a transducer may have many acoustical stacks therein, such as several thousand arranged in the 2D array. A known problem in ultrasound transducers using standard half wavelength thickness (λ/2) ceramic piezoelectric materials within the acoustical stack is the perturbation from the back of the acoustical stack, such as radiation losses, parasitic reflections and the like. To address this problem, a quarter wavelength thickness (λ/4) piezoelectric material has been used and is coupled with a high impedance layer that is positioned at the rear-facing part of the piezoelectric material. The high impedance layer is often referred to as a “dematching layer”. This arrangement induces a decrease in insertion losses in the 1 to 3 dB range, and also induces an 8 to 10 percent bandwidth (BW) increase (the rear “blocking” condition is similar to a symmetrical loading of the piezoelectric material, resulting in a lower mechanical Q). These advantages are coupled with a reduction of the input impedance of the transducer in the magnitude of 50 percent. In other transducers, a high impedance backing layer has also been used with a polyvinylidene fluoride (PVDF) piezoelectric material in order to decrease insertion losses and increase BW.
Unfortunately, problems occur when the transducers are used at some frequencies. For example, when the transducers are operating at frequencies above 5 MHz, the ceramic and dematching layer substrate properties and the joining material there-between together severely limit the mechanical action of the dematching layer. Also, the theoretical prediction of the expected performance enhancement resulting from the addition of the dematching layer is based upon the acoustical and mechanical properties of the two materials, and assumes a direct contact there-between across the surfaces of the dematching and ceramic layers. However, it has been very difficult to ensure direct contact between the dematching and ceramic layers, leading to rejection of assembled materials due to unacceptable performance.
Therefore, a need exists for improved acoustical stacks used within ultrasound transducers.